The invention relates to a method of identifying a TRAU frame in a mobile telephone system comprising a network part and at least one subscriber terminal having a radio connection to the network part, the subscriber terminal normally transmitting to the network part a speech frame to be placed in the TRAU frame, and when discontinuous transmission is used, the subscriber terminal transmits to the network part a SID frame to be placed in the TRAU frame, and on the basis of the sum of the bits in a SID code word, the TRAU frame is identified in the network part as a speech frame, a SID frame or a SID frame corrupted on the radio path.
The problem in the above arrangement it that a SID frame that is transmitted by a subscriber terminal and corrupted on the radio path may be erroneously interpreted as a speech frame. This causes interference towards the listener of the telephone on the side of the fixed network in the form of an unpleasant kind of xe2x80x9cclickingxe2x80x9d sound.
The problem arises because said identification is badly designed in the GSM specifications and cannot cope well with SID frames that are corrupted on the radio path as a result of interference.
In accordance with the GSM specification, when changing over from normal transmission to discontinuous transmission, a SID frame is transmitted immediately after the last speech frame. If the SID frame is corrupted on the radio path, it is interpreted as a speech frame, and the interference is heard by the listener as a xe2x80x9cclickxe2x80x9d.
It is the object of the invention to provide a method and an apparatus implementing the method for solving the above problems. This is achieved by a method of the type presented in the introduction, characterized by summing up, in addition to the SID code word bits, the other TRAU frame bits that normally contain speech, except for the SID parameter bits representing background noise.
The invention also relates to a mobile telephone system comprising a network part and at least one subscriber terminal having a radio connection to the network part, the subscriber terminal normally transmitting to the network part a speech frame to be placed in the TRAU frame, and when discontinuous transmission is used, the subscriber terminal transmits to the network part a SID frame to be placed in the TRAU frame, and on the basis of the sum of the bits in a SID code word the TRAU frame is identified in the network part as a speech frame, a SID frame or a SID frame corrupted on the radio path. The mobile telephone system of the invention is characterized in that a channel codec is arranged to sum up, in addition to the SID code word bits, the other TRAU frame bits that normally contain speech, except for the SID parameter bits representing background noise.
The preferred embodiments of the invention are disclosed in the dependent claims.
The invention is based on utilizing all the 260 bits transmitted and set to zero by a subscriber terminal. In addition to the SID code word bits, these include bits normally containing speech information, except for the SID parameter bits which represent background noise.
The method and system of the invention provide several advantages. The unpleasant interfering sounds are eliminated. No frame is unnecessarily rejected, resulting in improved speech quality for the listener.